Numerical Model of Reduction of Apparent Thermal Inertia by Interrupted Vertical Heat-Flow

Introduction: The JAXA Hayabusa2 sample return mission investigated asteroid (162173) Ryugu via remote sensing (Tsuda et al., 2013, Watanabe et al., 2019), deployed the DLR/CNES MASCOT (Ho et al., 2021), performed an artificial impact experiment (Arakawa et al., 2020) and returned samples to Earth (Yada et al., 2022). Ryugu is a rubble-pile asteroid with similarities to aqueously altered carbonaceous chondrites, in particular CI chondrites (Kitazato et al., 2021, Hamm et al., 2022, Nakamura et al. 2022, Yokoyama et al., 2022). One of the biggest surprises was the prevalence of boulders and dm-sized pebbles on the surface and the deficiency of smaller particles (Jaumann et al., 2019, Sugita et al., 2019). Such finer particles were expected to dominate the surface based on thermal inertia estimates from telescopic infrared observations (Mller et al., 2011). The MASCOT radiometer MARA and the main spacecrafts TIR infrared imager confirmed the thermal inertia estimates from telescopic observations despite the boulder-dominated surface (Grott et al., 2019, Okada et al., 2020). The low thermal inertia was confirmed to be an intrinsic property of the boulders themselves (Grott et al., 2019, Hamm et al., 2020, Sakatani et al., 2021, Hamm et al., 2022). More specifically, the presence of a layer of dust masking the thermophysical properties of the boulder was limited to small patches of thin dust layers, or no dust at all (Biele et al., 2019, Hamm et al., 2023). In contrast to these in-situ results, the analysis of sample fragments by lock-in thermography resulted in a much higher thermal inertia more comparable to that of meteorites samples (Ishizaki et al., 2023). In this study we attempt to reconciliate the results from spacecraft observations and laboratory analysis by expanding our thermophysical mode to incorporate horizontal fractures. This procedure has been proposed by Elder et al., 2022. We investigate if it is possible to explain the MARA observations with a fracture boulder of higher bulk thermal inertia. This work has implication on the regolith gardening on asteroids like Ryugu as weak and porous boulders would respond to impacts of micro-meteorites differently than fracture boulders with low porosity (Cambioni et al., 2021).Methods: We start from the 1D-thermal model as used in Hamm et al., 2020. The heat conduction equation is solved for a 1D grid of N points from x0 = 0 to xN . At the lower boundary condition, the flux is set to zero. The upper boundary condition is given by the energy balance. Illumination is calculated by averaging over those DEM-facets of the boulder shape model within the MARA field of view. The emissivity of the surface reduced by thermal reradiation as described in Hamm et al., 2023. Here we modify the model such that the heat conduction equation is given by:cp T/t = (T4(xd,t)- T4(xu,t))For xu